This application claims the priority of German Application No. 197 42 445.7, filed Sep. 26, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a process for regulating or controlling the braking power in the engine braking operation of a supercharged internal combustion engine.
The engine braking power is composed of the traction power and the braking power, which is brought about by throttling the exhaust gas flow of the internal combustion engine. With increasing back pressure in the exhaust gas duct of the internal combustion engine, there is an increase in the cyclic gas work of the pistons, which must be produced, in order to push out the exhaust gases against the back pressure. Over and above the traction power, the engine braking power is increased by additional structural expenditures in that an actuator is disposed in the exhaust gas duct of the internal combustion engine and backs up the exhaust gas stream as a function of its adjusted position, that is, as a function of the remaining cross section of the passage.
German Patent document DE 195 16 971 A1 discloses a supercharged internal combustion engine, which has a pivotable exhaust gas flap in the exhaust gas duct or pipe. In the braking operation of the engine, the exhaust gas flap is swivelled as a function of the desired braking power and opens a reduced flow cross section for blocking the exhaust gas stream.
Furthermore, German Patent document DE 43 30 487 C1 discloses a supercharged internal combustion engine having an exhaust gas turbocharger. The turbocharger has an exhaust gas turbine with an adjustable turbine geometry, which is used in the braking operation of the engine as a back pressure brake and, by an appropriate rotation of a pivotable guide baffle part of the turbine, reduces the flow of exhaust gas impinging on the rotor disk of the turbine or, aside from split streams, blocks it.
Both known methods control the engine braking power by the position of the respective actuators in the exhaust gas duct, an increase in engine braking power being linked directly to an increase in the back pressure, that is, to the turbine inlet pressure. No provisions are made for controlling the braking power of the engine by a control parameter in a cyclic structure reacting on itself in the sense of a negative feedback.
An increase in the braking power of the engine by raising the back pressure is possible only to a limited extent in the case of supercharged internal combustion engines. In the engine braking operation, the supercharger operating limits are reached rapidly with the high turbine inlet pressure and the corresponding rotational speed of the exhaust gas turbo supercharger. If the turbocharger operating limits are exceeded, increased wear and possibly a destruction of the supercharger air compressor of the exhaust gas turbo supercharger will result.
Since the control of the braking power or a simple control with the adjusted position of the actuator in the exhaust gas duct are inaccurate control parameters, the exhaust gas turbo supercharger must be operated in engine braking operation within its operating limits at a clear distance from them. The attainable and usable braking power therefore is greatly limited.
It is an object of the invention to provide a process for controlling the braking power in the engine braking operation of a supercharged internal combustion engine, which enables the maximum braking power to be attained within the operating limits of the exhaust gas turbo supercharger.
Pursuant to the invention, this objective is accomplished by providing a process for controlling the braking power in the braking operation of a supercharged internal combustion engine, the exhaust gas of which is backed-up by an actuator in the exhaust gas duct before it enters an exhaust gas turbine of an exhaust gas turbine supercharger, whereby the engine has a performance graph, in which nominal parameters for a large number of operating points are stored electronically and used by an engine controller as input parameters for a comparison of nominal and actual values. The performance graph comprises an electronically stored compressor performance graph of the turbocharger air compressor, which represents the relationship between the mass throughput and total pressure relationship for different exhaust gas turbo supercharger rotational speeds. A plurality of operating lines for certain actuator positions and certain engine speeds, as well as for a constant turbine inlet pressure, are filed in the performance graph. A preferred actuator position is inferred from the performance graph and set, in that, as a controlling parameter, the corresponding turbine inlet pressure for different exhaust gas turbo supercharger rotational speeds, compressor mass flows and compressor pressure relationships is taken as a nominal value from the performance graph. The actual value corresponding to the braking power called for is determined proportionally from a maximum value of the turbine inlet pressure within the operating limits of the exhaust gas turbocharger. The maximum value is inferred as a function of the engine speed from a characteristic rotational speed line stored in the performance graph.
An advantage of the inventive control process is seen therein that it is possible to do without rotational speed sensors for detecting the rotational speed of the exhaust gas supercharger, and without pressure sensors for measuring the turbine inlet pressure, the results of the measurement being unreliable because of the extremely high stresses. With the information of the electronically stored performance characteristics of the compressor, the actual inlet pressure of the turbine can be determined precisely from the parameters of mass flow and total pressure relationship at the compressor, optionally with the aid of parameters that are easily measured such as the ambient pressure, the supercharger pressure and supercharger air temperature by the engine controller, as a function of the rpm of the internal combustion engine. The actual value for the nominal value/actual value comparison for controlling engine braking power is determined proportionally from the maximum value of the turbine inlet pressure, which is specified by the steady state characteristics of the rotational speed within the supercharger operating limits. The proportionally determined actual value cannot exceed the maximum value so that the maximum value can be specified close to the supercharger operating limits of, in each case, the existing engine speed. The largest possible back-up effect in the exhaust gas duct and, with that, the maximum braking power is reached, when the steady state characteristics of the rotational speed of the maximum turbine inlet pressure are close to the pump limit of the compressor in the performance graph.
Advantageously, operating lines for the engine braking operation, the fired-up operation and further types of operation of the engine, such as the exhaust gas recycling operation, are recorded in the performance graph. As a result, optimum operating behavior can be attained with little application effort for each type of operation of the engine. Moreover, a braking signal of a braking signal transmitter, such as a brake pedal, can be supplied to the engine controller for indicating the braking operation state of the internal combustion engine. If the braking signal transmitter, in accordance with an advantageous further development of the invention, generates a normalized braking signal as a function of its operating position, and the braking signal is used by the control unit for determining the actual value as a corresponding proportion of the maximum value, then the braking power can be requested continuously variably directly over the adjustment of the brake signal transmitter while the supercharger operating limits basically are observed.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.